The natural and also the simplest code for the transmission of a digital signal is the NRZ (Non-return to zero) code, also known as pure binary. There are known to be many other codes. Those more usually distinguished are the codes which modify the format of the digital signal and the codes which consist of adding additional bits, which increases the thruput but does not modify the format of the digital signal. Amongst the first codes, one can quote as an example the RZ code (Return to Zero), the AMI code (Alternate Mark Inversion), the CMI code (Coded Mark Inversion) and the Miller code. The second codes are of the nBmB type, in other words at n bits they make m bits to correspond with m&gt;n. They may be used to embody a precoding prior to a coding following one of the first above mentioned codes.
These codes are used to respond to specific needs such as the facility of retrieving the clock, the possibility of detecting transmission errors, and compatibility of the energy spectrum of the coded signal with the transmission support.
As regards optical fiber digital transmission systems using direct detection, the codes procuring the best possible sensitivity or, in other words, the codes authorizing the lowest mean optical power at the input of the photoreceptor for a given error rate on the regenerated binary signal, are the NRZ and RZ codes. This is why these codes are used in the principal transmission experiments seeking sensitivity performance. These experiments are referred to in the article by B. L. KASPER and J. C. CAMPBELL and entitled: "Multigigabit-per-second Avalanche Photodiode Lightwave Receivers" published in the Journal of Lightwave Technology, vol. LT-5, no 10, Oct. 1987, p. 1361.